Conventional photochromic compounds have at least two states, a first state having a first absorption spectrum and a second state having a second absorption spectrum that differs from the first absorption spectrum, and are capable of switching between the two states in response to at least actinic radiation. Further, conventional photochromic compounds can be thermally reversible. That is, conventional photochromic compounds are capable of switching between a first state and a second state in response to at least actinic radiation and reverting back to the first state in response to thermal energy. As used herein “actinic radiation” means electromagnetic radiation, such as but not limited to ultraviolet and visible radiation that is capable of causing a response. More specifically, conventional photochromic compounds can undergo a transformation in response to actinic radiation from one isomer to another, with each isomer having a characteristic absorption spectrum, and can further revert back to the first isomer in response to thermal energy (i.e., be thermally reversible). For example, conventional thermally reversible photochromic compounds are generally capable of switching from a first state, for example a “clear state,” to a second state, for example a “colored state,” in response to actinic radiation and reverting back to the “clear” state in response to thermal energy.
Dichroic compounds are compounds that are capable of absorbing one of two orthogonal plane polarized components of transmitted radiation more strongly than the other. Thus, dichroic compounds are capable of linearly polarizing transmitted radiation. As used herein, “linearly polarize” means to confine the vibrations of the electric vector of light waves to one direction or plane. However, although dichroic materials are capable of preferentially absorbing one of two orthogonal plane polarized components of transmitted radiation, if the molecules of the dichroic compound are not suitably positioned or arranged, no net linear polarization of transmitted radiation will be achieved. That is, due to the random positioning of the molecules of the dichroic compound, selective absorption by the individual molecules will cancel each other such that no net or overall linear polarizing effect is achieved. Thus, it is generally necessary to suitably position or arrange the molecules of the dichroic compound within another material in order to form a conventional linear polarizing element, such as a linearly polarizing filter or lens for sunglasses.
In contrast to the dichroic compounds, it is generally not necessary to position or arrange the molecules of conventional photochromic compounds to form a conventional photochromic element. Thus, for example, conventional photochromic elements, such as lenses for photochromic eyewear, can be formed, for example, by spin coating a solution containing a conventional photochromic compound and a “host” material onto the surface of the lens, and suitably curing the resultant coating or layer without arranging the photochromic compound in any particular orientation. Further, even if the molecules of the conventional photochromic compound were suitably positioned or arranged as discussed above with respect to the dichroic compounds, because conventional photochromic compounds do not strongly demonstrate dichroism, elements made therefrom are generally not strongly linearly polarizing.
It would be advantageous to provide photochromic compounds, such as but not limited to thermally reversible photochromic compounds, that can exhibit useful photochromic and/or dichroic properties in at least one state, and that can be used in a variety of applications to impart photochromic and/or dichroic properties.